Abstract

Many of the mountain ranges in western Montana, and the adjoining intermontane basins, are interpreted as chiefly the effects of block faulting like that in the Great Basin. The region was elevated above the sea in late Cretaceous or early Tertiary time; then followed a long period of crustal stability in which a great thickness of rocks was eroded. By Oligocene time the region had been generally reduced to a surface of moderate to slight relief. During the Oligocene and Miocene the drainage became sluggish or ponded, chiefly because of slow crustal movements that outlined the present basins and ranges. Areas corresponding approximately to the present basins became depressed, and in these accumulated the Tertiary “lake beds.” Areas of uplift corresponding to the present mountains were eroded and thus contributed land waste and volcanic ash to the “lake beds.” In the late Miocene or early Pliocene the surface comprised areas of older rocks that, except for scattered residual peaks and ridges, had been eroded to slight or moderate relief; and areas of the “lake beds” that formed gently sloping or level plains. Excluding the residuals, this surface is called here the Late Tertiary peneplain.

Further leveling of the older rock areas and deposition of the “lake beds” was interrupted by a general re-elevation of the region accompanied by greatly accelerated local crustal movements that relatively elevated the present mountains. These movements continued intermittently and with decreasing intensity through the Pliocene and, except for small displacements on some of the faults as late as the Recent epoch, ceased in early or middle Pleistocene. They are thought to constitute a distinct late stage of the Cenozoic mountain building.

During the halt in the uplift of the mountains, wide stream valleys as much as 1500 feet deep were eroded in the elevated and deformed peneplain. In the basins during this pause, called the Old Valley cycle, the “lake beds” were reduced to gently sloping plains collectively referred to as No. 1 Bench. With renewed uplift the more vigorous streams deepened their channels across the mountain blocks as fast as the surface rose and thus excavated narrow inner valleys or gorges. In this, the Present cycle of erosion, No. 1 Bench of the “lake bed” areas was, in most of the basins, dissected to a series of terraces. The faulting appears to be indirectly related to an axis of compression trending northwestward from Yellowstone National Park. Horizontal compressive forces moved opposite parts of a deeply buried layer of the earth's crust toward this axis. Relief from the compression raised the overlying layer thus causing tensional strains that were relieved by normal faulting and movements away from the plane of the axis.

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